1. Field of the Invention
The invention relates to a method for producing fiber-reinforced polymer compositions using a plasticating extruder and a plasticating extruder for carrying out the method.
2. Description of Related Art
A method and a plasticating extruder are disclosed in DE-C 40 16 784, in which the fiber strand enters the impregnating device and the strand exits after impregnation in a planar form. There is, however, little possibility that there will be good wetting of the fiber and the scattered material. The main problem, though, is that the individual fibers of the widely spread-apart fiber strand cannot be kept apart by the spreading lugs. The individual fibers are tied together in a compact strand by the 90.degree. deflection from the guide transversely to the screw axis at the feed nozzle, up to the end of the impregnating channel and to the screw element of the extruder axis. This means that this compact strand, which comprises many endless individual fibers, is of a great thickness, thus leading to rapid pinching off or cutting off between the screw flights and the barrel wall. The resultant fiber entanglements, which are still not completely impregnated, are then very difficult to break up again with the mixing and kneading to be mixed with the remaining melt, or the mixing and kneading/shearing zone must be made so intensive and/or long that a very high proportion of very short fibers/fines are produced in the product as a result.
Furthermore, this results in more rapid pinching off of the compact fiber strand and uncontrolled slipping of the individual fibers. Different slipping rates of the glass fiber strands occur, which makes it extremely difficult or impossible for the machine to be in a state capable of carrying out the process, that is, undefined drive ratios and uncontrollable process states occur. Moreover, the pre-impregnated fiber strand has to be distributed homogeneously in the melt with the individual fibers while still in the mixing-in zone. The individual fibers pre-impregnated with liquid polymer are drawn in by the respective screw element and thereby cut or pinched off. As a result, the drive force that can be exerted on the fiber strand is relatively small, because it is not possible for an adequately large angle of wrap around the extruder shafts to occur. This also produces a great undefined slippage of the fibers in the feeding region, for which reason the fiber content in the polymer is subject to strong fluctuations. Measuring the changes in rate at the fed-in fiber strand and using the rotational-speed control on the extruder shafts to compensate for this error leads to a strong harmful pulsation in the molten polymer fed in the impregnating region and to pulsation in the mixing region and consequently in the product discharge. The envisaged melt control by the meter through the displaceable screw barrel does not solve this problem either, because when individual fibers slip considerably the rotational speed at the extruder shaft would have to be increased and more matrix melt would inevitably be conveyed from the melt located in the region of the screw. That is, the mixing ratio of fiber to matrix in this case changes disadvantageously. Furthermore, apart from the exact weight distribution of the glass fiber content, a pulsation-free even product discharge is required for product processing and material reproducibility.
The difficulties suggested in the preceding are not intended to be exhaustive but are among many tending to reduce the desirability of the known methods and the known plasticating extruders. Other noteworthy problems may exist; however, those presented above should be sufficient to demonstrate that those known methods and apparatuses are amenable to worthwhile improvements.